In a cellular radio access network such as an evolved universal mobile telecommunications system terrestrial radio access network (E-UTRAN), mobile devices such as user equipment (UEs) may typically communicate with other mobile devices through one or more network nodes such as evolved node Bs (eNBs). However, in view of public safety considerations, it may be desirable that fixed and/or mobile devices of such networks be configured to support direct communication between mobile devices under various circumstances, such as public emergencies or service outages. In order to enable such direct communication, mobile devices may arranged to exchange discovery signals, and discovery intervals may be defined during which the discovery signals are to be exchanged. Although the timing of the discovery intervals may be defined according to a common timing reference, some mobile devices may not be able to obtain that common timing reference and may operate in an asynchronous mode. In conventional systems, if they are to ensure that they transmit discovery signals within time intervals during which synchronous-mode devices will be monitoring the wireless channel, asynchronous-mode devices may be forced to transmit discovery signals continually, at significant cost in terms of power consumption.